Remoistenable adhesives

ABSTRACT

Paper coated uniformly with discrete dots of remoistenable adhesive, especially based on starch or polyvinyl alcohol, has curl stability comparable with particle gum coated papers. Preferably, the dots are less than 0.3 mm in diameter and typically spaced 0.5 to 1 mm (centers) apart. The product is made by screen coating dots of aqueous adhesive mix onto base paper.

This application is a continuation of application Ser. No. 914,497,filed Oct. 2, 1986, abandoned.

This invention relates to remoistenable adhesives and to paper webscoated with remoistenable adhesives.

Remoistenable adhesives are a class of adhesives which are normally nottacky and are rendered tacky (activated) by contact with water.Conventional gummed labels, adhesive postage stamps and some types ofgummed paper tape make use of remoistenable adhesives. Typically suchremoistenable adhesives are based on gelatinized starch adhesivesdeposited to form a film of adhesive on a paper web. Continuous films ofremoistenable adhesives, especially gelatinized starch are sensitive tochanges in humidity in that as the humidity increases the starch absorbswater and swells. The paper substrate also absorbs water resulting indimensional changes but to a different extent than the starch. Thiscauses the adhesive coated paper to curl to an extent dependent on theambient humidity. For many commercial applications, especially flatlabels, this curl instability is undesirable.

Two techniques are in current use to try to obtain remoistenable gummedpapers with acceptable curl stability. The first method is to coat thepaper with a continuous film of adhesive and subsequently tomechanically break up the film. This latter step is known as gumbreaking and is carried out by passing the coated paper over the edgesof blades with the adhesive layer on the outside. Usually a pair ofblades at right angles are used in succession, the paper passingdiagonally over each to give a diamond shaped pattern of breaks in theadhesive film, although this improves the humidity stability of theproduct but does not generally give a flat product. The second method isto deposit the remoistenable adhesive as a layer comprising a matrix ofvery fine particles on the substrate. This can be done by using anon-aqueous coating fluid in which the adhesive particles are notsoluble. To bind the adhesive particles to the substrate the coatingincludes a binder such as polyvinyl acetate. The overall effect is thatof a "honeycomb" of adhesive particles stuck to each other by the binderthe layer having a substantial void volume. One such method is describedin published European patent application No. 0041842. These adhesivesare known as particle gums and most commonly use gelatinized starch asthe remoistenable adhesive. The second method generally produces betterresults than gum breaking but is more expensive in materials andequipment.

Remoistenable adhesive coated papers having little or no tendency tocurl when the ambient humidity is changed i.e. papers that have goodcurl stability, are commonly described as being "flat". Among flatpapers varying degrees of "flatness" may be recognized. A major reasonfor requiring good flatness in remoistenable, especially label, paper isthat it, especially the non-gummed side, is often printed. A flat papercan be printed fairly readily, in contrast to a paper with poor flatnesswhich is liable to curl substantially and be difficult to feed into aprinting press, whether web fed or sheet fed. In extreme cases, as forexample with papers having a continuous film of gum, the paper can"tube" i.e curl to such an extent as to form a tube-like tangle which iseffectively unusable as a substrate for printing. Commercially availablegum-broken products may not tube but will normally not be flat enough tomake printing them a straightforward matter. Solvent coated particlegummed label paper is usually flat enough to print but is relativelyexpensive to manufacture.

This invention is based on an approach to the problem from a differentdirection in that it starts with an aqueous coating mix of theremoistenable adhesive and applies it to the paper web to give a coatingof discrete dots. Using this technique we have been able to makeproducts which are exceptionally flat, in some cases flatter even thanthose obtained using particle gums, but without the difficulty andexpense of using non-aqueous solvents.

The present invention accordingly provides a paper web coated uniformlyover one surface thereof with a discontinuous coating of a remoistenableadhesive in the form of discrete dots of the adhesive deposited directlyonto and adherent to the paper.

The invention includes a paper web coated uniformly over one surfacethereof with a discontinuous coating of a remoistenable adhesive in theform of screen coated, discrete dots of the adhesive deposited directlyonto and adherent to the paper.

It is a particular feature of the invention that the adhesive coating onthe paper is in the form of discrete dots. By describing the dots asdiscrete we mean that each dot adheres to the paper and is separate fromand not adhered to adjacent dots. In particular it is desirable inachieving optimum curl stability that, so far as is consistent with thesize of dots and the coatweight, which are discussed below, each dot isas far as possible from its neighbours. The practical way we have foundto do this is to coat the paper with a substantially regular array ofthe dots of adhesive. Thus, the use of regular arrays of dots for thecoating forms a particular feature of the invention. The precisegeometrical arrangement of dots is not of itself critical. However,arrays with high symmetry are advantageous as, for dots of the same sizeand overall number per unit area, they enable higher minimum distancesbetween nearest neighbours. Square, offset square and, especially,hexagonal arrays are particularly efficient in this regard.

The dots of adhesive are deposited directly onto and are adherent to thebase paper. In other words, the adhesive adheres to the base by virtueof its own adhesive properties and not those of a separate binder phase.As is described below, the process of the invention takes advantage ofthe remoistenable nature of the adhesive i.e. that it is tacky when wet,to coat the adhesive as an aqueous coating mix thus adhering it to thepaper. It is a particular advantage of the invention that the areas ofbase paper surface lying between the dots (after allowing for dotspreading) are effectively undisturbed and we believe that thiscontributes substantially to the good curl stability that can beobtained.

The products of the present invention can be made to be flat so thatprinting them is not restricted by curling. However, if a sheet of papercoated with large dots of remoistenable adhesive were printed on itsnon-adhesive surface, the presence of the (large) dots could cause alocalized increase in printing pressure over the dots thus leading to avariation in image density. This variation is undesirable and can beavoided by keeping the dots small. It is a particular feature of theinvention that the average dot diameter is not more than 0.5 mm and theuse of dots having diameters not more than 0.3 mm is particularlybeneficial. Because, as is noted below, the adhesive coatweight variesdirectly with the size of the dots, the dot diameter will usually be atleast 0.05 mm and more typically at least 0.1 mm. A particularly usefulrange is 0.1 to 0.25 mm.

In the present context dot "diameter" refers to the diameter of a circlewith equal area. It is desirable that the dots are circular orapproximately circular but may be triangular, rectangular or higherpolygonal e.g. hexagonal, especially with rounded vertices, orelliptical or annular or other distorted but near circular shape. Toremain near circular the ratio of maximum to minimum section through thecentre of the dot will not usually be more than 2 and will normally beless than 1.5. We have obtained satisfactory results with square andcircular dots. In practice, in manufacture the surface tension of thecoating mix will tend to round off the dots. Clearly the height of thedots would be expected to have a bearing on the printingcharacteristics. We have found that larger dots can be deposited to bethicker than smaller dots. We have been able to print the uncoated sideof coated paper of the invention, especially where the dots have anaverage diameter of not more than about 0.3 mm, line, full tone, blockand half tone images on conventional printing e.g. offset ("wet")lithographic, presses without experiencing any difficulties arising fromcurl instability or pressure differentials from the discontinuous natureof the coating. Registration did not seem to be a problem and goodmulticolour images were also printed successfully. The coated side ofthe paper has also been successfully monochrome and multicolour printedand, because the coating does not have the porous honeycomb-likestructure of particle gums, the printing ink film stays on the surfaceand gives a brighter, sharper and glossier image than is obtained onthose particle gum papers which can be printed on their adhesive coatedsides. Further, particle gummed papers are known to be liable todusting, because the adhesive particles are only relatively lightlybound (use of more efficient binding would tend to "blind" theremoistenable adhesive), whereas the dot coated products of thisinvention seem less prone to dusting than the uncoated base paper. Ineffect, the dots of adhesive improve the surface binding of the paper,presumably because they are directly deposited on and adhere to the basepaper surface. The adhesive coated side of the dot coated product can beprinted with line, full tone, block and half tone images. However, asthe ink generally forms a film over the surface of the coated paper,dots lying under the ink film are effectively blinded. Thus, printingblock full tone images may cause the coated paper to be non-adhesive inthose areas. Half tone imaged areas normally remain remoistenable but onremoistening the ink may be smudged.

An overall quantitative limit on the coating is provided by thefunctional requirement that the amount of adhesive, i.e. the coatweight,is at least sufficient to function effectively as a remoistenableadhesive. The minimum amount of adhesive needed for adequatefunctionality depends on the particular nature of the adhesive but willusually be at least 2 g m⁻² and typically at least 4 g m⁻². Foradhesives based on synthetic polymers such as acrylics the coatweightcan be in the range 4 to 6 g m⁻² and for polyvinyl alcohol 5 to 18especially 8 to 14 g m⁻². Starch based adhesives will typically require6 to 20 particularly 10 to 15 g m⁻². The upper limit on adhesivecoatweight is technical in that it will not be so much that the dots ofadhesive coalesce to form a continuous film and economic in thatgenerally no more adhesive than is needed to provide the requiredproduct performance will be used.

However, we have found that there is a relationship between dot size andcoatweight. Thus, changing only the dot size, it seems that with largerdots the dot can be made thicker giving a greater proportionate increaseof coatweight than might otherwise be expected. Other factors, includingthe properties of the adhesive coating mix used and the operatingconditions of the coater used also influence coatweight as also,plainly, does the dot to dot separation. As indicated above it isadvantageous to use small dots and to achieve relatively highcoatweights it is desirable to use high symmetry regular arrays with(absolutely) small dot to dot separation. Generally we have obtainedgood results using dot arrays having a ratio of dot diameter to minimum(edge to edge) dot spacing (in square or hexagonal arrays) of at least 1with typical values being in the range 1.2 to 2.2. At even higher ratiosit may be difficult to stop dot coalescence, which will prevent the dotsbeing truly discrete. Using such close spacings the proportion of thesurface of the paper under the dots is typically at least 25% andparticularly from 30 to 40%. Even with highly symmetrical regular arraysof dots using very much higher area coverage may lead to dot coalescenceor filming of the adhesive.

As has been indicated above, a wide variety of remoistenable adhesivescan be used in this invention including synthetic adhesives based onacrylic polymers, polyvinyl acetate or polyvinyl alcohol. The inventionis particularly applicable to remoistenable adhesives based on starch,modified starch and starch derivatives, and especially dextrins, becauseflatness, or rather the lack of it, is an especial problem with starchadhesives. The invention is also particularly applicable when theadhesive is based on polyvinyl alcohol (PVOH). PVOH based adhesives arerelatively more weight efficient i.e. less is generally needed toachieve a particular level of adhesion, than starch based adhesives. Asis known the adhesive properties of PVOH remoistenable adhesives varywith varying molecular weight. Low molecular weight materials give goodtack, high molecular weight materials give good adhesive strength andmedium molecular weight materials give moderate tack and adhesivestrength. High molecular weight PVOH's can give very high viscosities inwater at relatively low solids and this may limit the amount of suchmaterials used in the practice of this invention. Mixtures of adhesivescan be used to obtain desired product properties or processcharacteristics.

Although we have successfully used adhesive coating mixes consistingsolely of the adhesive polymer dissolved in water, it will be usual forthe adhesive polymer(s) to be formulated with other components. Weexpect that materials such as biocides, flavourings and sweeteners willbe included as desired according to the intended end use and thatprocess aids such as defoamers or materials to inhibit stringing (seebelow) will be used as necessary. Other possible additives includehumectants and plasticizers to protect the adhesive from excessivemoisture loss during processing or storage. However, quantitatively, themajor further components of the adhesive formulation will be clays, theuse of which is described in more detail below, and other polymericconstituents. The latter materials are exemplified by polyvinyl acetate(PVA) which can be included in starch/dextrin or PVOH based adhesives.The grades of PVA used in remoistenable adhesives will be those whichare compatible with the adhesive polymers used. Grades of PVA arecommercially available which are designed to be compatible with dextrinand PVOH remoistenable adhesives. The amount of PVA used in the adhesiveformulation will be selected to achieve the desired properties onsimilar criteria to its use in conventional remoistenable adhesives.Typically, when used, it will comprise at least 10% by and up to as muchas 80% but commonly 20 to 60% by weight of the polymeric components ofthe adhesive. Thus, although PVA is not by itself a remoistenableadhesive it can be the major (by weight) polymeric component ofpractical remoistenable adhesives. PVA will usually be provided to thecoating mix as a latex and this may give an adventitious benefit as thepresence of disperse phase particles (the PVA itself) will tend toreduce stringing (see below).

It is a particular feature of this invention that the adhesive coatingis of discrete dots. In the coating process, the dots of adhesivecoating mix will tend to spread between deposition on the paper websubstrate and drying. Generally, the higher the viscosity of the coatingmix, the less the dots of coating will spread. Higher viscosities can beachieved by using coated mixes using higher viscosity components e.g.higher molecular weight adhesives or adding thickeners, or by usingrelatively high solids contents in particular greater than 50% e.g. upto 70 or 75%. Such high solids contents also reduce the dryingrequirement and thus the interval between deposition and drying.Additionally, the coating mix will penetrate the base but, although somebase penetration is beneficial in adhering the individual dots to thebase, undue base penetration may take the form of sub-surface spread, inextreme cases leading to merging of adjacent dots under the papersurface and giving inferior flatness even though on surface inspectionthe dots of adhesive on the surface appear separate. As with surfacespread, spread by base penetration is reduced by using high viscosity,high solids coating mixes. A further advantage arising from the use ofhigh solids and thus high viscosity coating mixes is that the amount ofwater applied to the substrate web is small as is the extent to which itwets the web. This reduces the extent to which the web is affected byon-machine curl i.e. curl induced by the coating and drying processitself. This is advantageous in that it reduces the extent of on-machinedecurling e.g. by controlled application of water to the non-coated sideof the paper, necessary.

The coating of the product of this invention is deposited onto the paperweb substrate from an aqueous coating mix to produce discrete dots ofcoating mix on the web which are then dried. The dots are small and,especially where high solids coating mixes are used, the amount of waterapplied to the web is correspondingly small. Accordingly, care may beneeded to avoid overdrying the adhesive as such overdrying usually mayhave a deleterious effect on the operating characteristics of theadhesive. We have particularly noted this adverse effect in the rate ofremoistening and the rate at which the strength of the adhesive bondbuilds up. This can be exemplified by considering the adhesion of are-moistened label made from dot adhesive coated paper of this inventionto a paper substrate. A significant measure of the adhesion propertiesis the time taken for the bond strength to exceed the paper strength sothat an attempt to strip the label tears the paper of the label orsubstrate. Overdrying can substantially lengthen this time.

To our surprise we have obtained particularly good overall results inusing adhesive compositions containing substantial amounts ofhydrophilic clays e.g. china clay. The inclusion of small amounts e.g.up to 15% on a dry weight basis, of fillers such as clay in adhesiveshas long been known as economic extenders or rheology or processmodifiers. The conventional wisdom is that the use of larger proportionsof filler result in a marked deterioration in adhesive performance. Wehave found nothing to suggest that the conventional view is not correctfor conventional products especially where the adhesive is coated on thesubstrate as a film. However, in the context of this invention, theexpected deterioration is absent or at least much less marked than wouldbe expected. In some cases we have observed an improvement in thefunctioning of the product as a remoistenable adhesive when substantialamounts of clay are included as compared with when no clay is used.

We do not know why the inclusion of substantial amounts of clay does nothave the expected deleterious effect, but we think it possible that thepresence of the clay may make the adhesive less susceptible tooverdrying or that it facilitates penetration of water into the adhesiveduring remoistening. We think it likely that the maximum adhesivestrength of the adhesive will be less with clay present but, because thestrength of the adhesive bond is likely to be greater than that of thepaper substrate, this reduction is of little practical importance.

When used, the hydrophilic clay will be included typically as from 30 to70% and especially 40 to 60%, dry weight basis, of the adhesivecomposition. Suitable clays include china clay and other forms of kaolinand similar clays such as those sold under the trade name "Dinkie" byEnglish China Clays. The use of clays in this way can contribute to highsolids in the coating mix.

Accordingly, the invention includes a paper web, especially stock forprinted labels, coated uniformly over one surface thereof with adiscontinuous coating of a remoistenable adhesive, including ahydrophilic clay, in the form of discrete dots, in particular screencoated discrete dots, of the adhesive deposited directly onto andadherent to the paper.

We have found that the provision of the adhesive coating comprisingdiscrete dots can be carried out particularly effectively by using ascreen printer to coat the dots onto the paper web. In screen printingthe shape of the printed area and the amount of ink deposited on thesubstrate are determined by a compound screen/stencil. The "screen" wasoriginally of woven silk (hence "silk screen printing") but is now moreusually a metal wire or synthetic plastics mesh either woven or formedof a perforated sheet.

For the application of coatings in this invention a continuous rotaryscreen will usually be used. In conventional continuous rotary screenprinting, a stencil defines image areas with the rotary screen acting tometer the ink as in flat bed screen printing. In the present invention,the size of the discrete dots making up the coating and their spacingare comparable with typical rotary screen aperture size and spacing.Thus, a stencil will not usually be used to produce the dot coatedproducts of this invention. In practice, we have found it desirable touse screens with smaller and usually more closely spaced apertures thanare commonly used for screen printing on such equipment. Flat bedscreens e.g. hand screens used for laboratory work, usually haverelatively much finer screens than in continuous rotary screenequipment. Thus, if flat bed screens are used to produce the coatedproduct of this invention a stencil e.g. in the form of a resin layerimpregnated into the screen, defining the dot coating pattern willusually be used. The use of a screen, particularly a continuous rotaryscreen, to make the dot coated product of this invention is referred toherein as "screen coating".

The equipment used to carry out continuous rotary screen coatingtypically comprises a driven cylindrical screen, usually a perforated,thin walled metal cylinder, and a driven backing roll. The web to becoated passes over or round the backing roll and through the "nip"between the screen and backing roll. Although the term `nip` is used thecontact pressure is usually only adequate to maintain suitable contactbetween web and screen. A flexible squeegee blade, usually with a metaltip, is fitted inside the screen, with the blade tip pressed against theinside surface of the screen adjacent the length of the nip. Coating mixis introduced inside the screen to form a puddle between the squeegeeblade and the inside of the screen. Rotation of the screen pushes thecoating mix against the squeegee and screen and pushes it through thescreen with the squeegee blade acting to meter the amount. The squeegeeis adjustable to change the pressure applied to the blade and thus theforce at the tip. An increase of pressure increases the amount of mixpushed through the screen and thus the coatweight applied to the web.For constant dot spacing this will also increase dot diameter. Movementof the blade tip in relation to the nip can also affect coatweight. Theflexibility and size of the squeegee blade can also be changed but this,as is the choice of screen, is more a matter of setting up thanadjustment. We have also found that coatweight and dot size also tend toincrease with increasing line speed, presumably because the blade tendsto ride on a pool of mix which is pushed through the screen. As ismentioned above it is possible to vary the coatweight applied to thepaper web using a particular combination of screen and adhesive coatingmix by adjusting the operating conditions. This coatweight variationwill usually be reflected in a corresponding change of dot size. Caremay be needed when trying to increase the coatweight to avoid undueincrease in dot size as might lead to coalescence of the dots.

The technique of rotary screen coating places requirements on theadhesive coating mix to ensure good runnability. These requirements aresomewhat analogous to those for screen printing inks. The coating mixwill usually have a fairly high viscosity, typically in the range 1500to 5000 especially 2500 to 4000 cP Brookfield (Spindle No. 7 at 100 rpm)and will be moderately shear thinning. High viscosity at low shear helpsto prevent the mix oozing through the screen apertures too far inadvance of the squeegee blade. Under the squeegee blade the rate ofshear is higher and shear thinning helps to ensure good transmissionthrough the screen apertures under the blade and onto the paper web.After deposition on the web, the shear is again low, and restored highviscosity helps to prevent undue dot spread. If the mix weresubstantially thixotropic then the viscosity would remain low for asignificant time after the shear was reduced and this would tend topromote dot spread and possibly lead to coalescence which isundesirable. The selection or design of suitable adhesive coating mixesis relatively straightforward within these general requirements althoughsome trials may be needed to optimise conditions. The rheology of theadhesive coating mix can be varied by varying the solids content of themix, higher solids contents generally give higher viscosities; by choiceof the molecular weight of the adhesive used, higher molecular weightsgenerally give higher viscosities; by choice of adhesive type or byadding viscosity modifiers e.g. high molecular weight polymers such asalkyl cellulose derivatives, to increase mix viscosity or low molecularweight polar molecules, such as urea, or lower molecular weight polymerse.g. low molecular weight PVOH, to reduce mix viscosity.

In addition to mix rheology, the behaviour of the adhesive coating mixat the film split on the outgoing side of the nip between paper web andscreen is important. As the screen is rotating fairly rapidly and theadhesive mix is relatively viscous there can be a tendency for the mixto form "strings" or to "spin". This is analogous to the behaviour ofsome screen printing inks. Mixes with this tendency are described as"long" and can be "shortened" by using lower molecular weightconstituents or by incorporating phase boundaries into the mix e.g. byincluding emulsion droplets e.g. polymer latices, solid particles e.g.of clay or chalk, or incompatible compounds or polymers e.g. sodiumalginate for dextrins. As might be expected, stringing will usually beworse under higher shear i.e. at higher machine speeds. The rheology andspinning characteristics of the coating mix need be less closely definedif coating is carried out using a flat bed screen coating devicealthough the same general principles apply.

The invention, accordingly, includes as a specific feature a method ofmaking a paper web coated uniformly over one surface thereof with adiscontinuous coating of a remoistenable adhesive, which comprisesscreen coating an aqueous coating mix of the remoistenable adhesivethrough a screen or screen/stencil combination which provides imageareas in the form of discrete dots, whereby the adhesive is coated ontothe substrate as discrete dots adhered to the substrate and drying thecoated substrate.

As a primary product for which this invention is especially useful inlabels, the invention specifically includes a method of making a printedlabel carrying a coating of a remoistenable adhesive which methodcomprises screen coating an aqueous coating mix of the remoistenableadhesive through a screen or screen/stencil combination which providesareas of coating (`image` areas) in the form of discrete dots, wherebythe adhesive is coated onto the substrate as discrete dots adhered tothe substrate and drying the coated substrate and subsequently printingthe substrate to provide the label.

The following Examples illustrate the invention. All parts andpercentages are by weight unless otherwise indicated. Brookfieldviscosities were measured using a No. 7 spindle at 100 revs per minute(1.67 Hz) at or adjusted to 23° C. Ferranti-Shirley viscosities wereobtained by using a Ferranti-Shirley viscometer to produce rheogramsover a range of speeds. These data were mathematically transformed intoplots of viscosity against shear rate and viscosity values for low shear(shear rate=1000 sec⁻¹) and high shear (shear rate=5000 sec⁻¹) arequoted to indicate the shear thinning behaviour of the adhesive mixes.

In assessing the flatness of the adhesive coated papers two methods wereused as follows:

(a) 10 cm diameter circular samples of the paper are cut and exposed totest humidities of 40, 50 and 60% relative humidity (RH). Afterconditioning at each test humidity for 1 hour the extent of curl of thesamples is assessed.

(b) 19×14 cm rectangular samples (3 replicates) of the paper are cut andcycled from 70 to 30 to 70% RH in steps of 10% RH. After conditioningfor 1 hour at each test humidity the curl is assessed by measuring theheight above the horizontal support surface of each of the corners.Results are quoted as the maximum average corner height (in mm) and themaximum change in corner height.

The method (b) gives numerically more pessimistic results. Both methodstend to overstate curl for laboratory coated sheets as these are notde-curled on-machine. In method (b) the change in corner height figureprovides some compensation for this.

    ______________________________________                                        Proprietary Materials used in the Examples                                    Trade Name or                                                                 Designation                                                                              Description       Supplier                                         ______________________________________                                        C23        aqueous maize dextrin,                                                                          Laing-National                                              70% solids, Brookfield                                                        viscosity 18000 cP.                                                Nadex 442  aqueous maize dextrin                                                                           Laing-National                                              40% solids, Brookfield                                                        viscosity 10000 cP.                                                Gohsenol   Solid polyvinyl alcohol                                                                         Nippon Gohsei                                    GLO 5      (low molecular weight)                                             Gohsenol   Solid polyvinyl alcohol                                                                         Nippon Gohsei                                    GMO 14     (medium molecular weight)                                          Vinamul 8455                                                                             63% solids aqueous                                                                              Vinyl Products                                              polyvinyl acetate latex                                            Dinkie A   kaolin china clay (100%)                                                                        English China                                                                 Clays                                            Vinamul 83007                                                                            63% solids aqueous                                                                              Vinyl Products                                              polyvinyl acetate latex                                            Gohsenol   solid polyvinyl alcohol                                                                         Nippon Gohsei                                    GH 17      (medium molecular weight)                                          EDW 90     aqueous white farina                                                                            Helias & Co.                                                dextrin, 70% solids,                                                          Brookfield viscosity                                                          18000 cP.                                                          Gohsenol   solid polyvinyl alcohol                                                                         Nippon Gohsei                                    GM 14L     (medium molecular weight)                                          37-LAC-19  100% solid yellow dextrin                                                                       Avebe                                            Vinamul 8330                                                                             60% solids aqueous                                                                              Vinyl products                                              polyvinyl acetate latex                                            ______________________________________                                    

EXAMPLE 1

A wood free paper of basis weight 65 g m⁻² was dot coated using alaboratory screen and rubber squeegee with C23 (aqueous dextrin). Thescreen was of polyester monofiliment fibre mesh with 130 fibres cm⁻¹ andan open area of 30%. The stencil overlay, produced by selectivephoto-curing of a suitable resin impregnated into the screen, providedan array of circular holes at 19.75 lines cm⁻¹ and an open area of 50%.The circles were arranged in a square array with a separation (centre tocentre) of about 0.5 mm. The adhesive was coated to give a drycoatweight of 8 g m⁻². The dots in the dried coated paper had an averagediameter of about 0.3 mm and an average height of 0.025 mm.

The coated paper had good adhesive properties when remoistened andadhered to an uncoated sheet of the base paper. Curl testing by method(a) showed no discernible curl over the humidity range of the test. Acontrol circle coated with a continuous film of the same adhesive at 8 gm⁻² was tubed.

EXAMPLE 2

Example 1 was repeated with the following variations:

Adhesive: Nadex 442

Screen: mesh as in Example 1; stencil: square pattern of circular dotsat 10.75 lines cm⁻¹ and 50% open area.

The coating produced was of circular dots having an average diameter of0.66 mm an average separation of 1.1 mm and an average height of 0.036mm. The dry coatweight was 9 g m⁻².

The coated paper had good adhesive properties when remoistened andadhered to an uncoated sheet of the base paper. Curl testing by method(b) gave a maximum average corner height of 12 mm and a maximum changeof 10 mm. A control sample coated with a continuous film of the sameadhesive at 9 g m⁻² coatweight was totally tubed and became worse atlower RH.

EXAMPLE 3

Example 1 was repeated but using the screen used in Example 2. Thecoating produced was of a square array of circular dots having anaverage diameter of 0.7 mm, separation of 0.9 mm and height of 0.05 mm.The dry coatweight was 15 g m⁻².

The coated paper had good adhesive properties when remoistened andadhered to an uncoated sheet of the base paper. Curl testing by method(a) showed no signs of curl.

EXAMPLE 4

Example 1 was repeated with the following variations:

Adhesive: mix at 40% solids in water, Brookfield viscosity ca. 9000 cPof (% dry basis):

GM 14: 15%

GL 05: 15%

Vinamul 8445: 25%

Dinkie A: 45%

by dispersing the GM 14 in water at ca. 10% solids adding and dispersingthe GL 05, mixing in the Vinamul 8445 and then the Dinkie A to 40%solids.

Screen: mesh as in Example 1; stencil: square pattern of square dots at19.75 lines cm⁻¹ and 55% open area.

The coating produced was of circular dots having an average diameter of0.25 mm, height of 0.2 mm and separation of 0.5 mm. The coatweight was10 g m³¹ 2. The adhesive properties of the coated paper were good, theproduct gave a good time to tear (see test described below for Examples7 to 14). Curl stability was assessed qualitatively as good (comparableto Example 7 below) but not tested numerically. The low molecular weightPVOH gave the product excellent tack properties.

EXAMPLE 5

Example 1 was repeated with the following variations:

Adhesive: A blend was prepared consisting of:

90 parts (wet) of Vinamul 83007; and

10 parts (wet) of Gohsenol GH 17 dissolved at

15% solids in water,

to give an adhesive blend of 49% solids in water with a Brookfieldviscosity of 9600 cP at 23° C.

Screen: As in Example 2.

The coating produced was of circular dots having an average diameter of0.66 mm, an average height of 0.035 mm and an average separation of 1.15mm. The dry coatweight was 13 g m⁻².

The coated paper had excellent adhesive properties when remoistened andadhered to an uncoated sheet of the base paper.

Curl testing by method (b) gave a maximum average corner height of 35 mmand a maximum change of 7 mm. This example illustrates that, althoughthe coating and drying process in this particular case gave considerableinitial curl arising from the coating process, the dot coating ensuredthat the sheet was substantially stable to subsequent changes inrelative humidity. A control sample coated with a continuous film of thesame adhesive blend at 13 g m⁻² was tubed and became worse at lower RH.

EXAMPLE 6

Example 1 was repeated with the following variations:

Adhesive: EDW 90

Screen: As in Example 2.

The coating produced was of circular dots having an average diameter of0.81 mm, an average separation of 1.14 mm and an average height of 0.042mm. The dry coatweight was 10 g m⁻².

The coated paper had good adhesive properties when remoistened andadhered to a sheet of the base paper.

As in the previous example, the dot coated sheet exhibited curlresulting from the coating and drying processes. In order to simulateon-machine decurling, samples of the adhesive coated paper weresubjected to dampening of the uncoated side. This was achieved bydrawing a squeegee blade covered with a moist cloth over the papersurface. After subsequent drying the sheets were found to be virtuallyflat. The samples were subjected to curl testing by method (b). Thesheets gave a maximum average corner height of 9 mm and a maximum changeof 8 mm. A control sample coated with a continuous film of the sameadhesive at 10 g m⁻² coatweight and decurled in the same manner, curledsubstantially and became tubed at low RH.

The following Examples 7 to 14 were all carried out at a trial using acontinuous rotary screen manufactured by Stork X-cel BV of Boxmeer,Netherlands. Adhesive compositions A to E were used. The constituents,mix compositions, screens, coating conditions, product properties andtest results are set out below.

    ______________________________________                                        Adhesive Mix formulations A to E used in Examples 7 to 14                     Mix       Component        % dry                                              ______________________________________                                        A         Gohsenol GM14L   10                                                           Vinamul 8330     30                                                           Dinkie `A`       60                                                           defoamer         trace                                              B         37-LAC-19        47.5                                                         Dinkie `A`       47.5                                                         urea             5                                                  C         37-LAC-19        70                                                           Dinkie `A`       30                                                 D         37-LAC-19        98                                                           sodium alginate  2                                                  E         37-LAC-19        31                                                           Vinamul 8330     63                                                           triacetin        6                                                            biocide          trace                                                        flavouring and sweetener                                                                       trace                                                        defoamer         trace                                              ______________________________________                                    

                  TABLE 1                                                         ______________________________________                                        Properties of Adhesive Mixes A to E                                           Solids     Viscosity    Ferranti-Shirley (cP)                                 Mix   %        Brookfield (cP)                                                                            Low Shear                                                                              High Shear                               ______________________________________                                        A     40       3200          900     550                                      B     67       3500         1200     900                                      C     50       3500         2300     1800                                     D     53       --            500     360                                      E     48       --           3500     450                                      ______________________________________                                        Screens P to S used in Examples 7 to 14                                                                        Aperture                                             Mesh           Open Area diameter                                     Screen  (Lines inch.sup.-1)                                                                          (%)       (mym)                                        ______________________________________                                        P       40             16        266                                          Q       60             10        141                                          R       70             12        132                                          S       80             12        116                                          ______________________________________                                    

All these screens take the form of perforated, thin walled metalcylinders 537 mm in circumference and 0.66 L m long (to match thecoating width of 0.6 m). The apertures are arranged in a regularhexagonal array with the outer face of each aperture being hexagonal andslightly tapering inward (through the wall) towards a circle. `Openareas` are based on the relative areas of these circles and thediameters given are calculated from the manufacturer's quoted mesh andopen area data. `Mesh` in lines per inch are measured along a closestcentre to centre line of the aperture array. It will be recognised thatthe three dimensional shape of the apertures will give dots in thecoating having a slightly greater diameter than that quoted for theapertures even if no further dot sptread occurs.

                  TABLE 2                                                         ______________________________________                                        Coating Conditions.                                                                                  Speed                                                  Ex. No.                                                                              Mix     Screen  (m.min.sup.-1)                                                                         Notes                                         ______________________________________                                         7     A       Q       50       24 mm squeegee blade                           8     B       S       50                                                      9     B       S       100                                                    10     B       R       50                                                     11     C       P       30                                                      12a   C       Q       100      Low blade pressure                             12b   C       Q       75       High blade pressure                           13     D       P       15       *                                             14     E       Q       15                                                     ______________________________________                                         *In Example 13 the Dextrin on its own gave spinning problems. The             inclusion of the sodium alginate improved this to some extent but the lin     speed was still restricted.                                              

                  TABLE 3a                                                        ______________________________________                                        Description of Coated products                                                Dot                        Area      Ct. wt.                                          diam.   separation height                                                                              Covered Dry                                  Ex. No. (mym)   (mym)      (mym) (%)     (gm.sup.-2)                          ______________________________________                                         7      254     169        14    33       7                                    8      211     107        10    40      12                                    9      215     103        18    42      --                                   10      252     111        20    44      15                                   11      523     112        32    62      22                                    12a    347      76        18    61        14.4                                12b*   374      (50)      17    (71)    16                                   13                                       17                                   14                         20            10                                   ______________________________________                                         *Note:                                                                        the coating as somewhat filmed so that separate dots were not obtained.       This run is for comparison.                                              

Dot diameters and separation data were obtained from scanning electronmicrographs. The separation figures are edge to edge measurement. Thecentre to centre measurement is given by the sum of the diameter andseparation. Dot heights were measured as the difference in caliper(thickness) of dot coated and uncoated base paper. The figure representsan average of the peak heights of the dots. These data indicate theextent of dot spread (c.f. data on screens above).

In the trial sufficient dot screen coated paper was made in Examples 7,8 and 11 for it to be subsequently on-machine decurled using a steamshower to apply water to the uncoated face of the paper web. Apart fromthe fact that the paper was reeled up on rewind of the screen coater andmoved to a separate machine for decurling (because the experimental setup did not permit in-line decurling) this is equivalent to in-lineon-machine decurling. With the exception of Example 12b all Examplesgave much flatter and stable coated paper than would be given by paperhaving a continuous film of remoistened adhesive on it. Example 12bshowed signs that, although the coating showed visually separate dots,the dots had started to coalesce to form a film and that the dots werenot discrete. Although they were not specifically de-curled samples fromExamples 10, 12a and 14 gave good curl stability results summarised forExamples 7 to 14 in Table 4a below. Curl data was obtained using method(b) described above. The samples were also tested to give a guide to theremoistening characteristics with the results set out in Table 4a below.The test measures the time in seconds between remoistening of a piece ofdot adhesive coated paper and application to a piece of base paper andthe time when peeling the two pieces of paper apart tears one of thepieces of paper (rather than the pieces separating in the adhesivelayer).

                  TABLE 4                                                         ______________________________________                                        Test Results                                                                                       Curl Stability                                                                           Adhesion                                      Ex. No.  Curl        (delta mm) (secs)                                        ______________________________________                                         7       flat (d)    --         25                                             8       flat (d)      12 (d)   30                                            10       flat (d)    --         25                                            11       --          20         60                                             12a     slight      --         50                                            13       --          --         45                                            14       --           6         45                                            ______________________________________                                    

Entries followed by (d) indicate that the coated paper was decurledbefore testing.

A de-curled sample of paper from Example 7 was tested for curl stabilityusing test method (b) against commercial particle gummed and gum broken(2 products one with a PVOH based adhesive the other with a dextrinbased adhesive) products. The results are set out in Table 4b below.

                  TABLE 4b                                                        ______________________________________                                                           Curl Stability                                             Sample             (delta mm)                                                 ______________________________________                                        Example 7           6                                                         Particle gum       10                                                         Gum broken (PVOH based)                                                                          20                                                         Gum broken (dextrin based)                                                                       48                                                         ______________________________________                                    

EXAMPLE 15

A reel of dot coated product made as described in Example 7 wason-machine decurled, as described above, and sheeted on a precisioncutting machine. No runnability problems were encountered duringsheeting. Conventional film coated gummed products have poor runnabilityon such sheeters and usually cannot be processed on them. Samples ofthese sheeted dot coated product and of sheets of conventional particlegum coated paper were printed, some on the `face` i.e. non-adhesivecoated, side and some on the coated side using conventional commercialsheet fed offset lithographic printing equipment. No runnabilityproblems were observed for the dot coated product, whereas the particlegummed paper had inferior runnability showing substantial dusting of theparticle gum which was picked up on the litho blanket.

The face side printed samples of the dot coated product showed no signsof print deterioration from differential pressure and were of an equalstandard of printing to the face side of the particle gummed paper. Theadhesive side printed samples of the dot coated product were superior tothose of the particle gummed product which suffered from the effects ofthe dusting noted above. In fact the adhesive side of the dot coatedproduct gave less dusting than both of the face sides in the trial. Weinfer from this that the dots of adhesive act to improve surfacebinding. The images on the dot coated product were sharper and glossierthan those on the particle gummed paper. It seems that the openstructure of the particle gum layer sucks the ink below the top surfaceof the adhesive effectively partially masking the print. Theseadvantages were also seen when multicolour adhesive printing wastrialled.

What is claimed is:
 1. A coated paper having improved curl stability, comprising a paper web coated uniformly over one surface thereof with a discontinuous coating of a remoistenable adhesive based on a member of the group consisting of starch, a modified starch, and a starch derivative, in the form of discrete dots of the adhesive deposited directly onto and adherent to the paper, wherein a major portion of said one surface remains uncoated by said adhesive dots.
 2. Coated paper as claimed in claim 1 wherein the discrete dots of the remoistenable adhesive form a regular array.
 3. Coated paper as claimed in claim 2 wherein the regular array is selected from the group consisting of a square array, an offset square array, and a hexagonal array.
 4. Coated paper as claimed in claim 1 wherein the dots of remoistenable adhesive have an average diameter of from 0.05 to 0.3 mm.
 5. Coated paper as claimed in claim 1 wherein the dots of remoistenable adhesive have a shape selected from the group consisting of circular, hexagonal and square shapes.
 6. Coated paper as claimed in claim 1 wherein the adhesive includes from 30 to 70% by weight of the adhesive of a hydrophilic clay.
 7. Coated paper as claimed in claim 1 having two sides and having an image printed on at least one side thereof.
 8. Coated paper as claimed in claim 1, wherein the amount of surface area of said one surface coated by said adhesive is from about 30 to about 40%.
 9. Coated paper as claimed in claim 2, wherein the dot array has a ratio of dot diameter to minimum dot spacing of at least
 1. 10. Coated paper as claimed in claim 9, wherein the ratio of dot diameter to minimum dot spacing is from about 1.2 to about 2.2.
 11. Coated paper as claimed in claim 1, wherein said adhesive has a coatweight of from 10 to 15 g m⁻².
 12. A coated paper having improved curl stability, comprising a paper web coated uniformly over one surface thereof with a discontinuous coating of a remoistenable adhesive based on a member of the group consisting of polyvinyl alcohol, polyvinyl acetate, and acrylic polymers in the form of discrete dots of the adhesive deposited directly onto and adherent to the paper, wherein a substantial portion of said one surface remains uncoated by said adhesive dots.
 13. Coated paper as claimed in claim 12 wherein the discrete dots of the remoistenable adhesive form a regular array.
 14. Coated paper as claimed in claim 13 wherein the regular array is selected from the group consisting of a square array, an offset square array, and a hexagonal array.
 15. Coated paper as claimed in claim 12 wherein the dots of remoistenable adhesive have an average diameter of from 0.05 to 0.3 mm.
 16. Coated paper as claimed in claim 12 wherein the dots of remoistenable adhesive have a shape selected from the group consisting of circular, hexagonal and square shapes.
 17. Coated paper as claimed in claim 12 wherein the adhesive is based on polyvinyl alcohol and is coated at a coatweight of from 8 to 14 g m⁻².
 18. Coated paper as claimed in claim 12 wherein the adhesive includes from 30 to 70% by weight of the adhesive of a hydrophilic clay.
 19. Coated paper as claimed in claim 12 having two sides and having an image printed on at least one side thereof.
 20. Coated paper as claimed in claim 12, wherein the amount of surface area of said one surface coated by said adhesive is from about 30 to about 40%.
 21. Coated paper as claimed in claim 13, wherein the dot array has a ratio of dot diameter to minimum dot spacing of at least
 1. 22. Coated paper as claimed in claim 21, wherein the ratio of dot diameter to minimum dot spacing is from about 1.2 to about 2.2. 